1. Field of the Invention
The present invention relates to a shock absorbing type steering shaft which is capable of contracting in the axial direction upon collision of the car owing to the impact of the collision, and a method of manufacturing such steering shaft. Particularly, the present invention relates to a shock absorbing type steering shaft capable of stabilizing a collapse load which acts at the time of collision of the car even when the inner shaft of the car is a solid shaft so as to reduce the manufacturing cost, as well as a method of manufacturing such steering shaft.
2. Related Background Art
In a steering apparatus for a car, there is provided a shock absorbing type steering shaft which can be contracted in the axial direction upon collision, by the impact of the collision.
An example of a method for manufacturing such shock absorbing type steering shaft is disclosed in Japanese Patent Application Laid-Open No. 52-25330, in which a solid inner shaft having a non-circular cross section in a substantially oval form is inserted into a tubular outer shaft having a non-circular cross section in a substantially oval form, the outer shaft is pressed by a pressing member inwardly from the outer side thereof in the radial direction to plastically deform the both shafts locally, and then both the shafts are pressure-fixed to each other elastically by the portions thus plastically deformed. In this manner, while a relative movement between the both shafts in the axial direction is prevented normally, the plastically deformed portions are caused to collapse at the time of collision of the car, whereby the inner shaft is moved into the outer shaft to contract the entire length of the steering shaft, thereby absorbing the shock.
However, according to the manufacturing method disclosed above, since the plastic deforming is conducted in the state that the solid inner shaft is inserted in the outer shaft, an amount of plastic deformation of the outer shaft is not sufficient, so that there is a possibility of backlash in the pressure contact and fixation of both the shafts.
In Japanese Patent Application Laid-Open No. 1-58373, in the state that a jig is inserted in a tubular outer shaft having a circular cross section, the outer shaft is pressed inwardly in the radial direction by the pressing member from the outer side thereof, so that the outer shaft is plastically deformed locally to have a non-circular cross section. Subsequently, the jig is removed, and the solid inner shaft having a circular cross section is inserted into the outer shaft, whereby the inner shaft is pressure-fixed to the portion of the outer shaft which has been plastically deformed to the non-circular cross section.
However, according to the manufacturing method disclosed above, it is required to once insert the jig into the outer shaft and, after the plastic processing, to remove the jig. Thus, the manufacturing process becomes complicated for such steps of inserting and removing the jig and, as a result, the manufacturing cost is increased.
Further, in Japanese Patent Application Laid-Open No. 9-272447, an inward protrusion is formed in the radial direction on a tubular outer shaft having a circular cross section with a female serration, an annular recess from which a serration has been removed is formed on the outer peripheral surface of a solid inner shaft having a circular cross section with a male serration, and the annular recess of the inner shaft is caused to engage with the inward protrusion in the radial direction of this outer shaft in such a manner that the both shafts are pressure-contacted and fixed with pressure to each other.
However, according to the manufacturing method disclosed above, it is required to remove a part of the serration from the male serration formed on the outer peripheral surface of the inner shaft and to form the annular recess, which may result in an increase in the manufacturing cost.
Further, according to Japanese Patent Application Laid-Open No. 10-181615 (which has been converted from an application for a Japanese Utility Model Registration filed in 1992), the tip end of a tubular outer shaft having a circular cross section and the tip end of a solid inner shaft having a circular cross section are plastically deformed to have non-circular (flat or elliptic) cross sections, respectively. On the other hand, inner portions other than the tip ends of the both shafts are maintained to have circular cross sections. Then, the inner shaft is inserted into the outer shaft, and the portion having the non-circular cross section of the outer shaft is strongly pressure-contacted and fixed to the portion having the circular cross section of the inner shaft and the portion having the circular cross section of the outer shaft is strongly pressure-contacted and fixed to the portion having the non-circular cross section of the inner shaft, respectively, whereby the both shafts are pressure-contacted and fixed to each other.
However, according to the manufacturing method disclosed above, the plastic deforming of the tip end of the outer shaft and the plastic deforming of the tip end of the inner shaft are conducted separately, so that the manufacturing process becomes complicated, which may bring about an increase in the manufacturing cost.
Further, in Japanese Patent Application Laid-Open No. 8-91230, like in Japanese Patent Application Laid-Open No. 10-181615, the tip end of the outer shaft and the tip end of the inner shaft are plastically deformed to have non-circular cross sections. However, as a step of this plastic deforming, the tip end of the inner shaft is inserted in the tip end of the outer shaft to be maintained in an overlapping state, and these both overlapping tip ends are pressed inwardly in the radial direction by the pressing members to be plastically deformed to have non-circular cross sections. Thus, the both tip ends are plastically deformed at a time to simplify the manufacturing process.
However, according to the manufacturing method disclosed above, the inner shaft is required to be hollow and, when the inner shaft is solid, even if the tip end of the solid inner shaft is inserted into the tip end of the tubular outer shaft and the both overlapping tip ends are pressed inwardly in the radial direction by the pressing members, the plastic deforming can not be conducted satisfactorily with respect to the solid inner shaft.
Further, according to Japanese Patent Application Laid-Open No. 10-147245, like in Japanese Patent Application Laid-Open No. 8-91230 mentioned above, the both tip ends are plastically deformed by the pressing members at a time in the state that the both tip ends of the outer shaft and the inner shaft overlap each other. It is further arranged to adjust the collapse load by adjusting the press load of the pressing members.
However, according to the manufacturing method disclosed above, like in Japanese Patent Application Laid-Open No. 8-91230, when the inner shaft is solid, the plastic deforming can not be conducted satisfactorily.
Incidentally, when the inner shaft is a solid shaft, it is comparatively difficult to plastically deform the both tip ends at a time in the state that the tip ends of the both shafts overlap each other, like Japanese Patent Application Laid-Open No. 8-91230. However, a part of the above Japanese Patent Application Laid-Open No. 8-91230 discloses a method for pressure-contacting and fixing the solid inner shaft and the tubular outer shaft to each other, as shown in FIG. 19.
A bottomed hole 103 is formed at the tip end 101a of the solid inner shaft 101 having a circular cross section with a male serration 102, and the tip end 101a of this inner shaft 101 is fitted in the tip end 104a of the tubular outer shaft 104 having a circular cross section with a female serration 105. These both overlapping tip ends 101a and 104a are pressed by the pressing members 106 and 106 from the upper and lower directions thereof so that the both tip ends 101a and 104a are plastically deformed to respectively have non-circular (flat or elliptic) cross sections, meanwhile the inner portions other than the tip ends of the both shafts 101 and 104 are maintained to have the circular cross sections.
After that, the pressing members 106 and 106 are removed and the inner shaft 101 is further inserted into the outer shaft 104, the tip end 110a of the non-circular cross section of the inner shaft 101 is strongly pressure-contacted and fixed to the inner portion of the circular cross section of the outer shaft 104, and the tip end 104a of the non-circular cross section of the outer shaft 104 is strongly pressure-contacted and fixed to the inner portion of the circular cross section of the inner shaft 101.
However, the drilling and processing works are required to form the bottomed hole 103 on the tip end 110a of the solid inner shaft 101, which may bring about an increase in the manufacturing cost.
While the outer shaft 104 is obtained by plastically deforming a tubular shaft, the inner shaft 101 is obtained by plastically deforming a solid shaft on which the bottomed hole 103 is drilled. Thus, the level of the plastic deforming is not always the same for the both shafts, and one of the shafts may be deformed excessively. As a result, the force of pressure-contact and fixation is different between the both shafts, so that it becomes difficult to stabilize the collapse load to act upon collision of the car.
The present invention has been conceived taking the circumstances mentioned above into consideration, and an object of the invention is to provide a shock absorbing type steering shaft which can stabilize a collapse load acting upon collision of the car and can reduce the manufacturing cost, as well as a method for manufacturing such steering shaft.
In order to achieve the above object, there is provided according to the present invention, a shock absorbing type steering shaft comprising a tubular outer shaft having a female serration and a solid inner shaft having a male serration, characterized in that:
after the outer diameter surface of the outer shaft is pressed inwardly in the radial direction to plastically deform an axially inner portion thereof, the inner shaft is inserted into the outer shaft so that the inner shaft is pressure-contacted and fixed to the plastically deformed portion of the outer shaft.
In order to achieve the above object, there is also provided according to the present invention a method for manufacturing a shock absorbing type steering shaft for fitting and fixing a solid inner shaft having a male serration in a tubular outer shaft having a female serration, comprising the steps of:
inserting the tip end of the inner shaft into the tip end of the outer shaft to hold the same in a state that these tip ends are overlapping each other;
pressing the outer shaft at which the both tip ends are not overlapping each other inwardly in the radial direction by means of a pressing member to plastically deform an axially inner portion thereof; and
removing this pressing member to further insert the inner shaft into the outer shaft, thereby pressure-contacting and fixing the inner shaft to the plastically deformed portion of the outer shaft.
As describe above, according to the present invention, the tip end of the inner shaft is inserted into the tip end of the outer shaft to be maintained in an overlapping manner, the inner portion of the outer shaft is pressed inwardly in the radial direction by the pressing members and is plastically deformed, and the inner shaft is pressure-contacted to be fixed to this plastically deformed portion of the outer shaft. That is, the outer shaft is plastically deformed while preventing the outer tube from being excessively deformed, by plastically deforming the inner portion of the outer shaft in the state that the tip ends of the both shafts overlap each other slightly. Consequently, it is possible to stably maintain the force of pressure contact and fixation of the steering shaft in its accomplished state. It is also possible to stabilize a collapse load which acts at the time of collision of the car. In addition, since a drilling step is unnecessary, unlike in the conventional method, the manufacturing cost can be reduced.
According to another aspect of the present invention, there is provided a method for manufacturing a shock absorbing type steering shaft in which a solid inner shaft having a male serration is pressure-fitted in and fixed to a tubular outer shaft having the female serration, comprising the steps of:
pressing and deforming a predetermined portion of the tip end of the outer shaft from the outside thereof in the radial direction to form the female serration portion; and
pressing and fitting the male serration portion of the inner shaft in this pressed and deformed portion to pressure-contact and fix the inner shaft thereto. There is also provided a shock absorbing type steering shaft which is manufactured by such method.